Es muss heißen:

Inventory::Inventory()
{
	slots = new Slot*[5];
}

und nicht

Inventory::Inventory()
{
	Slot** slots = new Slot*[5];
}

Mit inventory.addItem(); wird der Zeiger "bla" vom Typ "Potion" erstellt, und dann einem Slot zugewiesen.
Beim Zuweisen bricht das Programm jedoch kritisch ab, warum?

Offset: 000012cb

Items.h

#ifndef _ITEMS_H_
#define _ITEMS_H_

//***INCLUDES***//

//***CLASS Slot***//
class Slot
{
public:
	virtual void use() = 0;
};

class Potion : public Slot
{
public:
	void use();
};

#endif _ITEMS_H_

Items.cpp

//***INCLUDES***//
#include "Items.h"

//***ITEM CLASSES***//
void Potion::use()
{
	;
}

Inventory.h

#ifndef _INVENTORY_H_
#define _INVENTORY_H_

//***INCLUDES***//
#include "Items.h"

//***CLASS Inventory***//
class Inventory
{
public:
	Inventory();
	~Inventory();
	void addItem();

	Slot** slots;
}; extern Inventory inventory;

#endif _INVENTORY_H_

Inventory.cpp

//***INCLUDES***//
#include "Inventory.h"

//***GLOBAL***//
Inventory inventory;

//***CLASS Inventory***//
Inventory::Inventory()
{
	Slot** slots = new Slot*[5];
}
Inventory::~Inventory()
{
	delete slots;
}

void Inventory::addItem()
{
	Potion* bla = new Potion;
	slots[1] = bla;
}

main.cpp

#include "Inventory.h"

void main()
{
	inventory.addItem();
}